Modulation device

ABSTRACT

Modulation device for laser radiation, having at least one modulation means which can change at least in part the laser radiation which passes through the modulation device, the modulation device having splitter means which can split the laser radiation into at least two component beams of radiation, the device furthermore in the direction of beam propagation downstream of the beam splitter means having beam combining means which can recombine at least two of the component beams of radiation, and at least one modulation means being located between the beam splitter means and beam combining means such that at least one of the component beams can be changed by at least one modulation means such that the laser radiation which has been combined by the beam combining means or in the area of the beam combining means at least in a given area of space has the desired modulation as a result of the interference of at least two component beams.

BACKGROUND OF THE INVENTION

This invention relates to a modulation device for laser radiation,having at least one modulation means which can change at least in partthe laser radiation which passes through the modulation device.

Modulation devices of the aforementioned type have been known for a longtime. They can be used in different applications. Here for example laserprinters, laser television, or even workpiece machining by laserradiation are to be named.

A typical modulation means used in the past is a so-called GLVmodulator. Such a GLV modulator is operated in reflection. On itsreflecting surface it has a whole number of crosspiece-shaped segmentswhich are located parallel to one another and which can reflect thelaser radiation. Each of these segments can be tilted in a concertedmanner. Modulation occurs generally by two segments which are directlyadjacent to one another being tilted differently or one of the segmentsbeing tilted and the other being left in its original position so that asmall phase difference between the two adjacent component rays which areincident on these segments is formed by this different tilting of thetwo adjacent segments. This phase difference by direct interference inthe area of the modulator leads to the propagation characteristic of thelight which has been reflected by the modulator being able to be changedin a concerted manner.

The disadvantage here is the fact that intensely coherent light must bepresent for this purpose. In general, this is especially not the case inlaser diode bars due to the extension of the individual emission sourcesof such a laser diode bar in the direction of arrangement of theemission sources (in the slow axis). Furthermore, it is alsodisadvantageous here that by the aforementioned peculiarity of the lightemerging from a laser diode bar generally more than two adjacent,especially four or six segments adjacent to one another are illuminated,so that the resolution of such a modulation means is extremely poor.Furthermore, the individual states for the adjacent segments which aretilted toward one another and for the adjacent segments which are nottilted to one another, that is, the segments of a correspondingcomponent area of the GLV modulator, can only be inadequatelydistinguished when a laser diode bar or a stack of laser diode bars isused as the laser source.

An object of this invention is to devise a modulation device of theinitially mentioned type which is made more efficient, especially whenusing a laser diode bar or a laser diode stack as the laser lightsource.

SUMMARY OF THE INVENTION

The modulation device has beam splitter means which can split the laserradiation into at least two component beams of radiation, that thedevice furthermore in the direction of beam propagation downstream ofthe beam splitter means has beam combining means which can recombine atleast two of the component beams of radiation, and that at least onemodulation means is located between the beam splitter means and beamcombining means such that at least one of the component beams can bechanged by at least one modulation means such that the laser radiationwhich has been combined by the beam combining means or in the area ofthe beam combining means at least in a given area of space has thedesired modulation as a result of the interference of at least twocomponent beams. In such a device, the advantage is that by splittinginto two component beams corresponding to one another, the quality andthe resolution of the modulation are independent of the coherence of thelaser radiation used.

According to one preferred embodiment of this invention, the laserradiation at least in sections in a first direction, which isperpendicular to the middle direction of propagation, has a greaterdivergence than in a second direction, which is perpendicular to themiddle direction of propagation and to the first direction, theseparation into component beams taking place in the first direction.Especially when using a laser diode bar, the first direction correspondsto the greater divergence of the fast axis, conversely the seconddirection corresponds to the smaller divergence of the slow axis. Ifthus, the separation takes place in the first direction, and thus in thedirection of the fast axis, the change of the corresponding componentbeam will likewise take place in the direction of the fast axis so thathere in addition the greater coherence of the laser radiation in thefast axis direction is still used.

It can be provided that the beam splitter means be made as a prism,especially as an at least partially mirrored prism. Alternatively, thebeam splitter means could also be made as a partially transparentmirror.

Furthermore, it can be provided that the beam combining means be made asa prism, especially as an at least partially mirrored prism.Alternatively, the beam combining means could also be made as apartially transparent mirror.

According to one preferred embodiment of this invention, at least onemodulation means can change at least one component beam of radiationsuch that it undergoes phase concerted shifts of individual or allcomponent rays, especially by half the wavelength of the laserradiation. Here there is a clear difference from the existing art inwhich within the component beam a phase shift to one another wasimparted to adjacent component rays. Non-adjacent component rays of thesame component beam are provided with a phase shift to one another, buta phase shift is induced especially only in one of the two componentbeams by a modulation means so that only after combining the twocomponent beams, at the beam combining means or in the area of the beamcombining means or downstream of the beam combining means, is modulationcaused by interference. In this way, for the case in which themodulation means are made as a modulator which is to be operated inreflection, especially as a GLV modulator, two or four or six segmentsof the modulator which are adjacent to one another no longer contributeto a modulation point or to a modulation bit, but in the preferred caseonly one individual element does so. In this way of course theresolution with which the laser radiation can be modulated can begreatly increased.

It is alternatively conceivable for the modulation means to be made as amodulator which is to be operated in transmission.

Moreover, it is possible for the modulation means to be made as atwo-dimensional modulator with which laser radiation incident on it canbe modulated with respect to two directions which are essentiallyperpendicular to one another. In this way, two-dimensional informationcan be modulated onto the laser radiation and at least in areas can makeline-by-line scanning in a printing process or the like superfluous.

It is conceivable that the modulation device can use even athree-dimensional modulator with which laser radiation incident on itcan be modulated with respect to three directions which are essentiallyperpendicular to one another.

A modulation device including beam splitter means, modulation means andbeam combining means can be regarded as an interferometer. For amodulation device as claimed in the invention, thus with respect to thearrangement of the aforementioned elements to one another all knowntypes of interferometers, such as for example a Michelsoninterferometer, are suited.

According to one preferred embodiment of this invention, in thedirection of beam propagation downstream of the beam combining means,there is a diaphragm which can mask out parts of the laser radiationcorresponding to the modulation which is to be achieved. Here it can beprovided that in the beam propagation direction upstream and/ordownstream of the diaphragm there are lens means, especially cylinderlenses which can image or focus the laser radiation onto the diaphragmand/or following the diaphragm can re-collimate the focussed laserradiation. Due to the fact that propagation of the recombined laserradiation in certain directions is enabled and in certain directions isprohibited by the interference caused by the modulation means in thearea of the beam combining means or downstream of the beam combiningmeans, a diaphragm is very well suited to masking out certain desiredportions of the laser radiation which for example correspond to a logic“0” when digital information is modulated on. Likewise, the portion ofthe laser radiation which is passed through the diaphragm willcorrespond to a logic “1”.

It can be provided that the laser radiation be divided into componentbeams, that subsequently at least one of the component beams isphase-shifted according to the modulation which is to be achieved, andthat subsequently the component beams are combined such that the desiredmodulation is achieved by interference of the two component beams. Thisprocess provides one skilled in the art with a method with which he canachieve very effective modulation of high resolution with simple means.In particular, this can take place by the aforementioned masking out ofparts of the combined laser radiation which correspond for example to alogic “0”. Furthermore, for the case in which the laser radiation of alaser diode bar in the direction of the fast axis is divided into twocomponent beams, as a result of introducing a phase shift in the fastaxis direction the quality of the modulation is greatly increasedcompared to the modulation process known from the existing art.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of this invention become clear from thefollowing description of preferred embodiments with reference to theattached figures.

FIG. 1 shows a side view of the modulation device as claimed in theinvention;

FIG. 1 b shows a view according to arrows 1 b-1 b in FIG. 1 a;

FIG. 2 a shows a detailed view of the modulation device as shown in FIG.1 a in a first state;

FIG. 2 b shows a view as shown in FIG. 2 a in a second state;

FIG. 3 a shows a schematic diagram which illustrates the relationshipbetween the intensity and propagation angle of the state in FIG. 2 a;and

FIG. 3 b shows a diagram which corresponds to FIG. 3 a and whichillustrates the state in FIG. 2 b.

DETAILED DESCRIPTION OF THE INVENTION

Laser radiation 1 which is incident on the modulation device can emergefrom a laser light source which is made, for example, as a laser diodebar. The laser light source in one direction, in FIG. 1 a and FIG. 1 bin the X direction, thus has a comparatively extended cross section withmany line emission sources which extend in the X direction and which arelocated next to one another. Furthermore, the laser light source whichis made as a laser diode bar in the direction perpendicular thereto,specifically in the Y direction, has a very short extension of, forexample, 1 micron. In this Y direction, which is called the fast axis,the divergence is much greater than in the X direction which is calledthe slow axis.

A modulation device as claimed in the invention is shown in FIG. 1 a andFIG. 1 b. The modulation device includes beam splitter means 2 which candivide the laser radiation 1 incident on it into two component beams. InFIG. 1 a, this division is illustrated by means of two arbitrarilyselected component rays 1 a and 1 b. The beam splitter means 2 in theillustrated embodiment, has two prisms 2 a, 2 b which are alike oneanother, lie on top of one another with two corresponding leg sides, andare cemented to one another. FIG. 1 a shows that the two component rays1 a, 1 b enter two separate halves of the beam splitter means 2,specifically two different prisms 2 a, 2 b. After reflection on therespective hypotenuse sides, they are reflected a second time on the legsides which are cemented to one another. The latter can be mirrored.Subsequently they emerge from the hypotenuse sides of the prisms 2 a, 2b such that they are deflected up and down roughly at an angle of 45° tothe original direction Z of propagation. In this way, the laserradiation is divided into two component beams which move way from oneanother up and down in FIG. 1 a from the beam splitter means 2.

The component beam which is moving up in FIG. 1 a is reflected on amirror 4 such that it is reflected down at an angle of −45° to the Zdirection. This is illustrated by the component beam 1 a which has beenselected by way of example. The component beam which has been deflecteddown, as is illustrated on the sample component ray 1 b, is reflected upby the modulation means 3 likewise at an angle of roughly 45° to the Zdirection. The modulation means 3 can be made as a GLV modulator. Inparticular, the modulation means 3 can have segments 5 which are locatednext to one another in the transverse direction, i.e. in the X directionin FIG. 1 b, especially crosspiece-shaped segments. Thecrosspiece-shaped segments 5 can reflect the light which is incident onthem, as is shown for the component ray 1 b. In particular, it ispossible for the tilt angle of the individual crosspiece-shaped segments5 to be changed such that the optical path of the component ray 1 b ismade larger or smaller by a short distance which can correspondespecially to the amount of half the wavelength of the laser radiation.The individual segments 5 are to extend over the entire width in the Xdirection in FIG. 1 b. Thus the corresponding segment 5 can be tilted ornot tilted in a concerted manner at a certain point in the X direction.In this way the component beams of the laser radiation 1 which areincident on the modulation means 3 may or may not be provided with aphase difference of half the wavelength in a concerted manner fordifferent X coordinates.

The modulation device furthermore includes a beam combining means 6which combines the component beams which have been reflected by themodulation means 3 and the mirror 4 so that the re-combined laserradiation 1 propagates in the positive Z direction in FIG. 1 a and FIG.1 b. This beam combination means 6 is made as a prism, on the external,optionally mirrored sides of which the component beams are reflectedsuch that they move in the positive Z direction after reflection. Thecomponent beams which have been recombined with one another in this wayunder certain circumstances according to the positions of the individualsegments 5 of the modulation means 3 have local phase differences of forexample half the wavelength of the laser radiation used.

Following the beam combining means 6, in the Z direction there are insuccession a cylinder lens 7 with a cylinder axis which extends in the Xdirection, a diaphragm 8 and another cylinder lens 9 with a cylinderaxis which likewise extends in the X direction. Here, as is apparentfrom FIG. 1 a, the diaphragm 8 is located at a distance from thecylinder lens 7 or from the cylinder lens 9, which corresponds ratherexactly to the focal lengths of these cylinder lenses 7,9, the focallengths of the cylinder lens 7 and the cylinder lens 9 being the same.Since in this way the two cylinder lenses 7, 9 are located at a distanceto one another which corresponds to twice the focal length, the laserradiation 1 which is parallel to the Z direction before entering thecylinder lens 7 will be parallel in turn to the Z direction afteremerging from the cylinder lens 9. The diaphragm 8 consists of twodiaphragm parts 8 a, 8 b which are located on top of one another in theY direction, the gap between them extending in the X direction. The gappresent between the diaphragm parts 8 a, 8 b is located essentiallyexactly on the focal line of the two cylinder lenses 7, 9.

FIGS. 2 a, 2 b show two different cases. In the first case, it isassumed that for the imaged parts of the laser radiation 1, thecorresponding segments 5 of the modulation means 3 were tilted such thatthe parts of the laser radiation 1 which have been reflected by themodulation means 3 have a phase difference of λ/2, i.e. of half thewavelength of the laser radiation used, relative to the correspondingparts of the laser radiation 1 which have been reflected by the mirror4. These parts of the laser radiation will thus not be able to propagateexactly in the Z direction after recombination in the beam combiningmeans 6 as a result of interference. This is illustrated in FIG. 3 a inwhich the intensity I in arbitrary units is plotted against angle θ, theangle θ indicating the angle between the Z axis and the direction ofpropagation of the laser radiation 1. FIG. 3 a which shows simply aschematic illustrates that in the direct Z direction no propagation ofthe parts of the laser radiation which are interfering with one anotheroccurs. This is shown in FIG. 2 by the laser radiation 1 which has beenfocussed by the lens 7 in the area of the diaphragm 8, thus in the areaof the focal plane, being focussed not in the XZ plane, but shortlyabove and shortly underneath the XZ plane. As a result of introducingthe diaphragm 8 into the radiation path thus laser radiation 1 which hasbeen modulated in this way will not emerge to the right, i.e. not in thepositive Z direction, from the diaphragm 8.

FIGS. 2 b and 3 b show parts of the laser radiation 1 in which thecorresponding segment 5 of the modulation means 3 has not been titled sothat these parts of the laser radiation 1 which have been reflected bythe modulation means 3 do not undergo a phase shift so that destructiveinterference after combination by the beam combining means 6 does notoccur. In this case FIG. 3 b illustrates that the propagation peak liesroughly in the Z direction. This case is also illustrated in FIG. 2 b inwhich the focal line of the laser radiation 1 which has been focussed bythe cylinder lens 7 lies essentially in the area of the diaphragm 8 inthe XZ plane. This results in that this part of the laser radiationpasses essentially unhindered through the diaphragm 8 and after passingthrough the second cylinder lens 9 propagates parallel to the Z axis inthe positive Z direction.

It is possible, instead of the beam splitter means 2, to use other beamsplitter means. They could be beam splitter means which correspondroughly to the beam combining means 6. Furthermore, instead of the beamcombining means 6 other beam combining means can also be used, forexample beam combining means which correspond essentially to the beamsplitter means 2.

It is also possible, instead of the modulation means 3 which is made asa GLV modulator, to use other modulation means. In particular it is alsopossible to use modulation means which can cause two-dimensionalmodulation of the light which is incident on the modulation means. Forexample, here the light emerging from the two-dimensional light source,such as the light of a stack of laser diode bars, can be modulatedaccordingly. What is important is simply that a component beam,specifically the component beam which has been deflected down in FIG. 1a, with the component ray 1 b which has been selected by way of example,is provided concertedly in individual component sections with a phaseshift. The individual component sections in which the phase shift iscarried out can be stipulated by information which is to be modulatedonto the laser radiation 1. The information can be for example printinginformation or also information for laser television or information formachining of a workpiece or the like.

It is furthermore possible to use a reflecting modulation means insteadof the mirror 4. What is important here is simply that between theindividual component areas of the separated laser radiation 1 whichcorrespond to one another a definable phase difference can be producedin order to allow individual component areas through the diaphragm 8 orto have them blocked by the diaphragm 8.

The description of the illustrated embodiment of the modulation deviceclearly shows that the principle of the modulation device is similar tothat of an interferometer.

It is possible to collimate the laser radiation 1 emerging for examplefrom a laser diode bar upstream or downstream or in the area of themodulation device with respect to its fast axis divergence and withrespect to its slow axis divergence with the corresponding means knownfrom the prior art. With respect to fast axis divergence they arecylinder lenses with cylinder axes which are aligned in the X direction.With respect to slow axis divergence they are arrays of cylinder lenseswith cylinder axes which are aligned in the Y direction.

1. In combination, laser radiation and a modulation device, A modulationdevice for laser radiation, the modulation device comprising at leastone modulation means which can change at least in part the laserradiation which passes through the modulation device, wherein themodulation device comprises beam splitter means which can split thelaser radiation into at least two component beams of radiation, that thedevice furthermore in the direction of beam propagation downstream ofthe beam splitter means comprises beam combining means which canrecombine at least two of the component beams of radiation, and that theat least one modulation means is located between the beam splitter meansand the beam combining means such that at least one of the componentbeams can be changed by the at least one modulation means such that thelaser radiation which has been combined by the beam combining means orin an area of the beam combining means at least in a given area of spacehas the desired modulation as a result of the interference of at leasttwo component beams, wherein the laser radiation has, at least insections, in a first direction (Y), the fast axis, which isperpendicular to the middle direction (Z) of propagation, a greaterdivergence than in a second direction (X) the slow axis, which isperpendicular to the middle direction (Z) of propagation and to thefirst direction (Y), the separation into component beams taking place inthe first direction (Y).
 2. The combination as claimed in claim 1,wherein the beam splitter means are made as a prism, or as an at leastpartially mirrored prism.
 3. The combination as claimed in one of claim1, wherein the beam splitter means are also made as a partiallytransparent mirror.
 4. The combination as claimed in claim 1, whereinthe beam combining means are made as a prism, or as an at leastpartially mirrored prism.
 5. The combination as claimed in claim 1,wherein the beam combining means are made as a partially transparentmirror.
 6. The combination as claimed in claim 1, wherein the at leastone modulation means can change at least one component beam of radiationsuch that it undergoes a concerted phase shift of at least one of itscomponent rays, by half the wavelength of the laser radiation.
 7. Thecombination as claimed in claim 1, wherein the at least one modulationmeans are made as a modulator which is to be operated in reflection, asa GLV modulator.
 8. The combination as claimed in claim 1, wherein theat least one modulation means are made as a modulator which is to beoperated in transmission.
 9. The combination as claimed in claim 1,wherein the at least one modulation means are made as a two-dimensionalmodulator with which laser radiation which is incident on it can bemodulated with respect to two directions which are essentiallyperpendicular to one another.
 10. The combination as claimed in claim 1,wherein an interferometer is formed by the beam splitter means, themodulation means and the beam combining means.
 11. The combination asclaimed in claim 1, wherein in the direction (Z) of beam propagationdownstream of the beam combining means there is a diaphragm which canmask out parts of the laser radiation corresponding to the modulationwhich is to be achieved.
 12. The combination as claimed in claim 11,wherein in the direction (Z) of beam propagation upstream and/ordownstream of the diaphragm there are lens means, cylinder lenses whichcan focus the laser radiation onto the diaphragm and/or following thediaphragm can re-collimate the focused laser radiation.